This invention relates to a system for performing surgery by vibrational heating and more particularly to a system for performing surgery with ultrasonic heating guided by magnetic resonance (MR) imaging.
Conventional Magnetic Resonance Imaging (MRI) provides the radiologist with internal views of a subject's anatomy. MRI can produce excellent contrast between different tissues and is useful in planning surgical procedures. A tumor in a subject is much more visible in an MR image than as seen in actual surgery because a tumor and normal tissue often look similar in surgery. The actual tumor may also be obscured by blood during surgery. A view of the heated region is provided with the use of MR temperature sensitive pulse sequences. Known MR temperature sensitive pulse sequences are described, for example, in D. LeBihan et al. U.S. Pat. No. 4,914,608 In-Vivo Method for Determining and Imaging Temperature of an Object/Subject from Diffusion Coefficients Obtained by Nuclear Magnetic Resonance, issued Apr. 3, 1990. Experimentation has shown that a heated zone above a critical temperature destroys living tissue. This zone increases in size with time, as the heat is applied, to reach a steady state of both temperature and heat flow. If the maximum temperature is limited to 100.degree. C., then the heated zone, i.e., the area exceeding a critical temperature causing destruction of tissue, approaches 1 centimeter in diameter. It is difficult to predict the heated zone geometry because the heat flow depends on the perfusion of blood as well as the tissue thermal properties.
Tumors have been selectively destroyed in cancer subjects using focused ultrasound heating in the absence of MR imaging at the University of Arizona, as reported by B. E. Billard et al., "Effects of Physical Parameters on High Temperature Ultrasound Hyperthermia", Ultrasound in Med. & Biol. Vol. 16, No. 4, pp. 409-420, 1990 and hereby incorporated by reference. Billard et al. disclose that the control of heat is improved by using short heating pulses where the effect of blood perfusion is negligible. However, since they do not image the temperature distribution, it is difficult to hit small, deep laying targets.
As indicated above, an ultrasound transducer produces a relatively small, intense focal region. Often the focal region of the ultrasound transducer is smaller than the tissue that requires treatment. Accordingly, the focal region must be moved across the morbid tissue to fully ablate the tumor. The focal region is moved by sweeping the treatable area with the focused ultrasound beam. Sweeping can be accomplished by mechanically moving the transducer relative to the patient, or vice versa, but such methods are cumbersome. An alternative method for sweeping the focal region relies upon phased array treatment wherein electronic circuitry drives an array of ultrasound transducers to sweep a phased array ultrasonic beam over the treatment area. However, phased array ultrasound devices are complex and hence expensive to fabricate and to operate.
Accordingly, a need exists for a relatively simple, efficient and economic apparatus for treating a larger volume of tissue without significantly changing the size of the transducer or requiring the complexity of a phased array transducer and phased array driving electronics.